1. Field of the Invention
The present invention relates to an alkaline storage battery such as nickel-hydrogen storage battery and nickel-cadmium storage battery and method of fabricating thereof. More particularly, the present invention relates to an improvement in an alkaline storage battery comprising an electrode block obtained by spirally winding a non-sintered electrode having a porous metal material provided with a three-dimensionally continuous network skeleton filled with an active material with an opposing electrode with a separator a provided interposed therebetween and a process for the production thereof.
2. Description of the Related Art
As a nickel electrode to be incorporated in an alkaline storage battery such as nickel-cadmium storage battery and nickel-hydrogen storage battery there has heretofore been used a so-called non-sintered electrode plate comprising a porous metal material (active material retainer) having a three-dimensionally continuous network skeleton filled with an active material. This kind of a porous metal material having a three-dimensionally continuous network skeleton has a porosity as high as about 95% and thus can be filled with an active material at a high density, making it possible to obtain a high capacity battery. At the same time, since this kind of a non-sintered electrode plate can be prepared by filling a porous metal material with an active material as it is, the necessity for a troublesome treatment for activation can be eliminated, making it easy to produce a storage battery. The treatment for activation comprises a step of generating electrode active material and a step of immersing the electrode active material.
In recent years, there has been a growing demand for the enhancement of battery capacity. This demand for the enhancement of battery capacity has been met by raising the packing of an active material. However, the increase of the packing of an active material leads to the increase of the packing density of the active material or the increase of the electrode plate that causes the electrode to harden. Thus, the electrode plate shows a drastically deteriorated windability when spirally wound, lowering the yield in production and the battery quality. Then, an approach which comprises providing an electrode plate filled with an active material with flexibility to improve the windability thereof has been proposed in JP-A-60-246561 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-62-136760, and JP-A-10-223215.
The methods proposed in the above cited patents comprise filling a porous metal material having a three-dimensionally continuous network skeleton with an active material, pressing the porous metal material at a predetermined pressure, and then passing the porous metal material through a leveler device provided with a number of rollers. By thus passing an electrode plate filled with an active material which has been pressed through a leveler, many cracks are produced on the surface of the electrode plate, enhancing the flexibility of the electrode plate and hence making it possible to wind the electrode plate smoothly.
In accordance with the methods proposed in the above cited methods, however, a porous metal material having a three-dimensionally continuous network skeleton filled with an active material which has been rolled is then subjected to roller treatment in the same direction as the rolling direction, causing cracks to be introduced in the direction perpendicular to the rolling direction. Accordingly, the direction of production of cracks is parallel to the winding axis. As a necessary consequence, the rolling direction of the electrode plate coincides with the direction of spiral winding.
Therefore, a load is always applied to the porous metal material having a three-dimensionally continuous network skeleton in the same direction at various steps of filling of active material, rolling and roller treatment. Accordingly, the three-dimensionally continuous network skeleton constituting the porous metal material undergoes plastic deformation resulting in work hardening. As a result, even if the electrode plate is subjected to roller treatment so that cracks are introduced into the electrode plate, it is difficult to improve the windability thereof.
In order to improve the windability of the electrode plate by introducing cracks into the electrode plate, it is necessary to provide the electrode plate with cracks at a very small pitch in the winding direction of the electrode plate. However, since the pores formed by the three-dimensionally continuous network skeleton in the porous metal material are stretched such that the resulting planar shape has a long axis in the rolling direction. The occurrence of pores which have been stretched to have a long axis makes it difficult to provide cracks at a reduced pitch and hence improve the windability of the electrode plate.
Therefore, the present invention has been worked out to solve the foregoing problems. The relationship between the rolling direction of the electrode plate filled with an active material and the direction of roller treatment was studied. As a result, it was found that when these directions are perpendicular to each other, the windability of electrode plate can be improved. The present invention has been worked out on the basis of such a knowledge. The first object of the invention is to obtain an electrode body having an improved windability of electrode plate and hence provide a high capacity storage battery having an excellent high rate dischargeability. The second object of the invention is to provide a method of fabricating an electrode body having an improved windability of electrode plate.
The present invention has been worked out to accomplish the foregoing objects. In order to accomplish the first object of the invention, the alkaline storage battery according to the invention has an arrangement such that pores formed by a network skeleton in the porous metal material on the plane thereof are stretched to have a planar shape similar to ellipsoid or deformed ellipsoid having a long axis, cracks are formed in the non-sintered electrode in parallel to the longitudinal direction of the pores, and the electrode plates are spirally wound in such an arrangement that the cracks are oriented parallel to the winding axis.
Preferably the cracks are arranged at a very small pitch in the crosswise direction of the pores.
Thus, when cracks are formed in parallel to the longitudinal direction of pores which have been stretched to have a shape similar to ellipsoid or deformed ellipsoid having a long axis, these cracks are oriented in the crosswise direction of the pores which have been stretched to have a shape similar to ellipsoid or deformed ellipsoid so that they are arranged at a very small pitch. Accordingly, by spirally winding this electrode plate on a winding axis parallel to these cracks, an electrode coil having an substantially round shape can be formed. As a result, the windability of the electrode plate can be improved, making it possible to obtain a high capacity and quality storage battery.
Further, cracks are formed at a very small pitch. The cracks are finely divided. At the same time, since the spiral electrode block is round to cause dispersion of structural pressure, shortcircuiting caused by cracking can be prevented. At the same time, since the electrode plate is wound in the crosswise direction of the pores formed by a network skeleton, the load applied to the porous metal material during winding can be reduced, inhibiting local elongation of the electrode plate and hence preventing the occurrence of defects such as break of network skeleton in the porous metal material. As a result, the collecting properties can be improved, making it possible to obtain a high capacity and quality storage battery.
Further, said cracks are arranged at a very small pitch in the crosswise direction of said pores.
Preferably said electrode block spirally wounded is formed so as to be real enhanced roundness.
Preferably, said porous metal body is made of formed nickel. In order to accomplish the second object of the invention, the method of fabricating an alkaline storage battery of the invention comprises a rolling step of a step of filling a porous metal material with an active material in the network skeleton thereof to form a non-sintered electrode and rolling the non-sintered electrode to a predetermined thickness, a roller treatment step of passing the non-sintered electrode which has been rolled to a predetermined thickness through a series of rollers in the direction perpendicular to the rolling direction at the rolling step so that cracks which are oriented at a very small pitch in the direction perpendicular to the rolling direction are formed in parallel to the rolling direction, and a winding step of winding the non-sintered electrode in such an arrangement that the cracks formed at the roller treatment step are oriented in parallel to the winding axis.
When the non-sintered electrode having a porous metal material filled with an active material in the network skeleton thereof is rolled to a predetermined thickness at the rolling step, the pores formed by the network skeleton are stretched to have a planar shape similar to ellipsoid or deformed ellipsoid having a long axis. Subsequently, when the non-sintered electrode is passed through a series of rollers so that cracks thus formed are oriented in the direction perpendicular to the rolling direction, the cracks are oriented in the crosswise direction of the pores which have been stretched to have a shape similar to ellipsoid or deformed ellipsoid. Thus, cracks are formed at a very small pitch.
Subsequently, when the non-sintered electrode is wound at the winding step in such an arrangement that the direction of cracks is parallel to the winding axis, winding can be easily effected, making it possible to form an electrode coil having a substantially round shape. At the same time, since the non-sintered electrode plate is wound in the crosswise direction of the pores, the load applied to the porous metal material during winding can be reduced, inhibiting local elongation of the electrode plate and hence preventing the occurrence of defects such as break of network skeleton in the porous metal material. As a result, the collecting properties can be improved, making it possible to obtain a high capacity and quality storage battery.
In the case where the weight of the porous metal material having a three-dimensionally continuous network skeleton is constant, as the number of pores formed by a network skeleton on the plane of the porous metal material per inch of length, i.e., PPI decreases, the diameter of pores increases and the pitch between cracks increases. On the contrary, as PPI increases, the diameter of pores decreases and the pitch between cracks decreases. Accordingly, when the present invention is applied to a non-sintered electrode provided with a porous metal material having a three-dimensionally network skeleton with PPI of 200 or less, a substantially round spiral electrode body can be obtained to exert a further effect.
Further, when an active material is packed at a high density, a high capacity storage battery can be obtained. However, when the packing density of active material is raised, the electrode plate hardens. On the other hand, when the packing density of active material is lowered, the electrode plate doesn""t harden, improving the windability thereof. In order to attain the effect that makes it possible to wind the non-sintered electrode in the crosswise direction of the pores formed by the network skeleton on the plane thereof, the packing density of active material is preferably 2.6 g/cm3-void or more taking into account the ability to enhance capacity and the windability. The term xe2x80x9cpacking density (g/cm3-void) of active materialxe2x80x9d as used herein is meant to indicate the weight of the active material packed per volume of void excluding the metal portion in the porous metal material provided with a three-dimensionally continuous network skeleton.
Preferably the process further comprises a step of cutting the wherein said porous metal material filled with said active material so as to have a predetermined width after rolling step and prior to the roller treatment step.